The present invention relates to cryogenic refrigerators, in particular, Gifford McMahon (GM) refrigerators, GM type pulse tube refrigerators, and Solvay refrigerators. Coldheads of such cryogenic refrigerators include a valve mechanism, which commonly consists of a rotary valve disc and a valve seat. There are discrete ports, which, by periodic alignment of the different ports, allow the passage of a working fluid, supplied by a compressor, to and from the regenerators and working volumes of the coldhead.
GM and Solvay type refrigerators use compressors that supply gas at a nearly constant high pressure and receive gas at a nearly constant low pressure. The gas is supplied to a reciprocating expander that runs at a low speed relative to the compressor by virtue of a valve mechanism that alternately lets gas in and out of the expander. U.S. Pat. No. 3,119,237 to Gifford shows an early pneumatically driven GM expander and a multi-ported rotary spool valve to control gas flow to the regenerator out of phase with gas flow to the drive piston. In a subsequent U.S. Pat. No. 3,205,668, Gifford discloses a multi-ported rotary disc valve that uses the high to low pressure difference to maintain a tight seal across the face of the valve. He states that this type of valve is superior to the spool type valve because the leak rate is lower, even after it has run a long time and has experienced some wear. This type of valve has been widely used in different types of GM refrigerators as shown for example in Longsworth U.S. Pat. No. 3,620,029, and Chellis U.S. Pat. No. 3,625,015.
This type of valve has the disadvantage of requiring an increased amount of torque as the diameter is increased to accommodate larger ports. Lobb, U.S. Pat. No. 4,987,743 describes a means of reducing the force on the face of the valve disc by having a plug in the back of the valve disc that is attached to the motor shaft. High pressure gas on the back side of the plug and low pressure gas in the cavity between the face of the plug and the central part of the back of the valve disc puts an axial load on the motor bearings, but reduces the force on the face of the valve disc. This results in a lower torque being required to turn the valve disc. The direction of force on the motor shaft is towards the valve seat.
Gifford also conceived of an expander that replaced the solid displacer with a gas displacer and called it a “pulse tube” refrigerator. This was first described in Gifford U.S. Pat. No. 3,237,421 which shows a pulse tube connected to valves like the earlier GM refrigerators. It also shows a pulse tube expander connected directly to a compressor so it pulses at the same speed as the compressor. This is equivalent to a Stirling cycle refrigerator.
Early pulse tube refrigerators were not efficient enough to compete with GM type refrigerators. A significant improvement was made by Mikulin et al., as reported in 1984, (E. I. Mikulin, A. A. Tarasow and M. P. Shkrebyonock, ‘Low temperature expansion (orifice type) pulse tube’, Advances in Cryogenic Engineering, Vol. 29, 1984, p. 629) and significant interest ensued in looking for further improvements. Descriptions of major improvements since 1984 can be found in S. Zhu and P. Wu, ‘Double inlet pulse tube refrigerators: an important improvement’, Cryogenics, vol. 30, 1990, p. 514; Y. Matsubara, J. L. Gao, K. Tanida, Y. hiresaki and M. Kaneko, ‘An experimental and analytical investigation of 4K (four valve) pulse tube refrigerator’, Proc. 7th Intl Cryocooler Conf., Air Force Report PL-(P-93-101), 1993, p 166-186; S. W. Zhu, Y. Kakami, K. Fujioka and Y. Matsubara, ‘Active-buffer pulse tube refrigerator’, Proceedings of the 16th Cryogenic Engineering Conference, 1997, p. 291-294; and J. Yuan and J. M. Pfotenhauer, ‘A single stage five valve pulse tube refrigerator reaching 32K’, Advances in Cryogenic Engineering, Vol. 43, 1998, p. 1983-1989.
All of these pulse tubes can run as GM type expanders that use valves to cycle gas in and out of the pulse tube, but only the single and double orifice pulse tubes have been run as Stirling type expanders. Stirling type pulse tubes are small because they operate at relatively high speed. The high speed makes it difficult to get to low temperatures so GM type pulse tubes running at low speed are typically used for applications below about 20 K. It has been found that best performance at 4 K has been obtained with the pulse tube shown in FIG. 9 of Gao, U.S. Pat. No. 6,256,998. This design has six valves which open and close in the sequence shown in Gao's FIG. 11.
In order to keep the valve disc in contact with the valve seat, in most of the prior art, the valve disc is located in a chamber, which is filled with high-pressure working fluid. Usually, this chamber is connected to the supply side of a compressor. This is shown in Gifford U.S. Pat. No. 3,205,668, Longsworth U.S. Pat. No. 3,620,029, Chellis U.S. Pat. No. 3,625,015, and Lobb U.S. Pat. No. 4,987,743.
Since the valve disc is in the chamber connected to the supply side of the compressor, the wear dust from the valve disc tends to be blown into the cold head itself, which degrades performance. The pulse tube refrigerator is more sensitive to the dust than a conventional GM refrigerator because this dust tends to stick on the surface of the needles which are used to adjust the opening of the orifices at the warm end of the pulse tube, or to accumulate in the orifices and flow passages. The performance of a pulse tube refrigerator is sensitive to the opening of the orifices, thus it is desirable to keep them free of dust. The tendency to blow dust into the ports on the valve disc is due to high pressure gas being on the outside blowing dust radially inward toward the low pressure regions in the center of the valve disc.
It has been found that less dust from the wear of the valve disc collects in the warm end of the regenerator and pulse tube orifices if the flow through the valve disc is reversed. That is to have high-pressure gas enter through the center of the valve disc face and discharge radially to low pressure on the backside of the valve disc. Asami et al. U.S. Pat. No. 5,361,588 shows an arrangement for a GM refrigerator where the high-pressure gas from the compressor acts against a valve seat to push it into the face of a rotary valve. A bearing is shown to hold the valve disc against the axial force of the valve seat, rather than transferring it as an axial load on the motor shaft. The flow of gas in this arrangement is reversed from the conventional arrangement shown in previous patents. Conceptually the conventional valve disc shown in FIG. 7 of Longsworth U.S. Pat. No. 3,620,029 could have the flow reversed if the spring that is shown would apply enough force to keep the valve disc seated.
It would be desirable to provide a valve unit that reduces the amount of dust blown into the cold heat and at the same time reduces wear on the valve and requires lower torque.